SLAM - the math AND the reality

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duraleigh said:
I will speculate and say you have some testing errors. You are right FC does not jump all over the place for no apparent reason. Back to the basics for a minute....

Chlorine is ONLY consumed by two things......organics in your pool or UV from the sun.

With that in mind, you cannot have a constant 5 ppm during a 10-12 hour daylight period....the sun will consume some of your FC.

The OCLT is fairly bulletproof. If there are organics in your pool, that will consume FC as well. It may do so in a more inconsistent manner than UV from the sun but not to the extent that you report.

I understand this makes many people very frustrated and even angry but the "laws of nature" are pretty darn consistent.
Click to expand...
I meant 5 FC, with SWG on. Sorry.
It was at 100% though , so not good.

I tested Sunday night and lost 2ppm, hence the SLAM starting yesterday afternoon.

Last night it was FC 26 and this morning FC 17. So I went from 2ppm to 9 ppm overnight loss, in 1 day.

Does that seem realistic ?

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2 ppm FC in 3 hours is higher than normal. Even for those who lose 4 ppm FC in 24 hours, you'd expect 2 ppm FC to be lost perhaps over 4-5 hours of daylight. You definitely have something consuming chlorine beyond just breakdown from sunlight (of course, if the CYA level were very low then the loss could be from sunlight, but that's not your situation).
 
ok, thanks. But still tracking way better than the 9 ppm overnight loss last night . I am just about to go into pool now and remove the light - it has to be done, even though I cannot isolate the power. I will assume if it is not waterproof then it would be blowing the breaker right now lol.
 
7/21/15

2pm
FC 25 (after adding more bleach to keep in 24-28 range)

At 1pm, I cleaned behind light (it seem ok, surprisingly). I do know I need a new one - it is 115v old, and it has 14 screws and clamps on the back holding the glass to the fixture, they will not undo , so I will have to replace the whole light. Something else to now go research. Hopefully there are standard diameters that just slot right into the hole already there. I can then just cut the cable where it enters the waterproof case, and wire in a new one.

Anyway, I also went around with a screwdriver and scraped about 15 pieces of green algae off of cement between numerous tiles. The cement just dissolved to the touch , so I guess another project is to get it all re-tiled.

Anyway, I presume the 6ppm loss over 3 hours after cleaning and scraping is most likely the FC eating up all the additional algae I dropped in pool, right ?

One last thing. I did not have time to take the main drain cover off, what is likelihood of it needing cleaning ? My algae does not seem to be on pool surfaces , just in some of the tile cement just above waterline.

thx
 
I've read that the main drain cover is a popular hangout for algae. It's your call. Maybe give it a night or two to see if you've reduced or eliminated the overnight loss?

I think you could try using a turkey baster to pump some straight bleach in there, then cover it with a weighted Tupperware for a bit so the circulation doesn't dilute it too quick?

Don't wait too long though, you should recognize if what you did changed the pool's chlorine habit.


Dom
 
Sounds like a plan. Will clean in tomorrow - I just dumped another gallon of bleach in to raise from 20 to 28 , so it will drop to 24-26 for tonight's last reading. Whilst it is probably safe to swim, I did so earlier to clean, and I could smell the bleach. Not nice!
 
Just one question and I apologize if it's been discussed, but I don't recall it. Have you turned your SWG off? You don't want to have it on during the SLAM? It will skew the numbers.




[emoji176] Lisa P.
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Welcome to TFP! :wave:

Yes that would be OK to have the SWG off at night for any Overnight Chlorine Loss Test (OCLT) and on during the day though it won't be able to keep up since the FC loss rate during the day is pretty high in absolute FC. If you turn up the % ontime of the SWG you just end up shortening its life some, but if you aren't going to be home to maintain the SLAM well then the SWG is better than nothing.
 
jerryandcora said:
So the SWG is off at night and on during the day to help maintain levels?
Click to expand...

I work from home for an internet social media company, so have the luxury of being able to take care of it regularly. Therefore, I switch the SWG off for the whole SLAM. I teat every 2-3 hours, brush everything 3 times a day and clean filter daily too. The main reason I keep it off, is because I like to record loss over time without anything compromising the data. That way, you get a pretty good indication of how things are going. For example, you might lose a total of 14 ppm, like I did yesterday, then 6 the next day then say 3 the next (at which point you are probably back to normal and should pass the overnight. It is not super accurate, but shows a trend at least.

- - - Updated - - -

7/22/15
6.15 am
FC 23

Nearly there. I will be done by end of today , I would imagine.
 
7/22/15

5pm
FC 21.5 (10ml method)
FC 24 (5ml method - which I actually prefer as less margin for error)

So 22.75 average. Well within normal levels , only dropped between 1 and 2.5 ppm in 5 hours of peak sunlight.

I think I am safe to let it drop now to normal levels and just measure overnight for next few days.
 
I wouldn't go below SLAM until you pass an OCLT. Chances are you'll pass, but why the rush to let the FC drop already?

I made sure I passed two. I don't want to ever have to worry about SLAMing again.

Dom
 
Because it is impossible to accurately test at SLAM levels to a 1ppm level. I have yet , in 20+ attempts , ever got the same result on the same sample when testing it 3 times at FC25. The margin of error makes it mathematically impossible .

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As long as the rate of loss is low and expected given the test error, it's OK to lower the FC somewhat to gain better precision for an OCLT assuming other criteria are still met. Also, see this post for some information on improving the accuracy of measurements. For the OCLT, the two biggest test errors are the sample volume measurement and getting a consistent drop size. Because the water sample size is measured with a line at the meniscus, it should have a lower error with a larger sample volume, so 25 ml vs. 10 ml. Of course, you'll use more reagent that way, but it should reduce at least one of the two errors. The drop size error will net out the same in absolute FC because it's a percentage error and while you use more drops, they represent proportionally smaller FC per drop so that cancels out.

The reason we say to pass the OCLT before lowering the chlorine level is to make sure that what needs to be killed or oxidized is done so that it doesn't come back. Lowering the FC to 20 ppm or somewhat below helps reduce test error. The other way to approach this is to still have the OCLT at your higher FC but if the criteria accounting for 5% of the FC level is still met, then you're probably in good shape and can confirm this at a lower FC level. So at 25 ppm FC a 5% error would be 1.25 so worst case OCLT could be 2.25 still "meeting" the criteria. In practice, error between two measurements (i.e. a difference measurement) isn't always biased in the same direction so if the true loss were 1 ppm then with the 5% error one may measure from a 0.25 gain to a 2.25 loss.

At the end of the day, you should be able to determine your own personal test error by just testing the water several times in a row. If your results are inconsistent, then that's due to your testing error assuming you are testing the same or similar water sample.
 

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